The cleanest way to size a main panel for a small office is to start with the actual load, not the guess of what “sounds about right.” I would list every expected load in the space: general lighting, receptacles, dedicated equipment like a copier or server rack, and any HVAC or water-heating loads if they are part of the office. For office spaces, lighting and receptacle loads are often estimated from square footage and expected use, but you still need to account for any fixed equipment at its nameplate rating.
A practical first step is to calculate the connected load in volt-amperes or watts. For general office lighting and receptacles, many designers use a load calculation method based on area and occupancy, then add the actual ratings of dedicated equipment. Computers and office electronics are usually treated as part of the receptacle load unless you have a dedicated IT area with known loads. HVAC is often the biggest piece, so the compressor, air handler, and any electric heat need careful attention. If there is electric heat, that can change the panel size a lot.
Once you have the estimated load, apply the demand factors allowed by your local code. In many office applications, not every load runs at full power at the same time, so the demand load can be much lower than the total connected load. That is why you should not simply add every breaker size together and assume that is the service size. After you calculate the expected demand, convert it to amperage by dividing by the system voltage. For example, on a 208/120V three-phase system, the math is different than on a 120/240V single-phase system, so the panel choice depends on the actual service.
Then choose a panel with enough main breaker capacity and spare spaces for growth. For a small office, it is usually smart to leave some headroom for future circuits, because tenants often add another printer, a break-room appliance, or extra receptacles later. A panel that is technically adequate but completely full on day one can become a headache fast.
Also check the service equipment upstream, not just the panel itself. The feeder, meter base, disconnect, and utility service all have to support the load. A larger panel does not help if the feeder or service size is still too small. Short-circuit rating and available fault current matter too, especially in commercial spaces.
If you are working from a tenant fit-out or remodel, I would strongly recommend having a licensed electrician or electrical engineer review the final load calculation before you buy equipment. The code requirements and utility rules can vary a lot by location, and a quick check now is much cheaper than replacing the wrong panel later.
